A transformer or coil with a low voltage and large current may be realized by using a metallic plate winding to reduce the number of manufacturing steps and to prevent bulkiness of the windings. (See, e.g., Japanese patent document JP-A-2001-155933.)
One application relates to a discharge lamp lighting circuit, such as a metal halide lamp, used in an illumination light source for a vehicle such as a motor car. DC-AC conversion may be obtained using a transformer for power transmission to the discharge lamp. The transformer is provided with a detecting terminal attached to a secondary winding or detecting winding in order to detect the voltage applied to the discharge lamp.
The voltage detected using the detecting terminal or detecting winding is used for power control of the discharge lamp or for detection of an abnormal state of the discharge lamp.
If the magnetic coupling within the transformer is not uniform in a lighting circuit for high-frequency lighting a discharge lamp in which the transformer is compact, it is difficult to detect the voltage accurately. This is attributed to the fact that the relationship between the turn ratio and voltage significantly deviates from linear as a result of the loose/dense state in the coupling within the transformer. Therefore, a correction circuit is required. This, however, may result in complicated circuit construction, upsizing and increased cost. For example, where the discharge lamp is employed as the illumination source for a motor car, the lighting circuit must be arranged within a limited space (e.g., where a lighting circuit unit is accommodated within a lamp).
One problem addressed by this disclosure is how to assure the accuracy of voltage detection in a transformer suitable for downsizing and high frequency, as well as a reduction in the number of components and cost.